1. Field of the Invention
The invention relates to a method and apparatus of motion detection which is particularly well suited to detect motion in a television signal which was obtained from a film-to-television converter using the conventional 2-3 pull-down technique.
2. Description of the Prior Art
Motion picture film is normally exposed at a rate of 24 frames per second. When reproducing these films in a television system with a frame rate of 25 frames per second (such as PAL or SECAM) it is universal practice to simply reproduce the 24-frame rate film at 25 frames per second. The small increase in the pitch of the reproduced sound and the slight reduction in the running time (i.e., speed-up) of the film have been a generally accepted compromise. The use of this method in a 30 frame per second television system (such as NTSC), however, is totally unacceptable. This 30 frame per second system uses a special method in which one motion picture frame is exposed for two television fields, whereas the next motion picture frame is exposed for three television fields. This method of alternately holding the film in the projector gate for either two or three television field exposures is called 2-3 pulldown.
Although 2-3 pulldown has worked well in the past, it can cause problems when digital signal processing methods are applied to the television signal. For example, many digital television systems require an accurate motion detector in order to select a proper signal processing algorithm, e.g., for deciding whether or not to use a line or frame comb filter for luminance/chrominance signal separation. Known motion detectors for this purpose generally compare two picture elements that are a frame (two fields) apart and base their motion detection decision on the difference in signal amplitude between these two picture elements. If the television picture is obtained from a motion picture film and a 2-3 pulldown has been used for the frame-rate conversion, it has been found, as will be explained in greater detail below, that the motion detector will fail regularly, which may lead to a degradation of picture quality. If, for example, the motion detection signal was being used for controlling the use of either a line or a frame comb filter in a television signal processor, a regularly occurring failure of the motion detector (i.e., an indication of no motion when in fact there is motion) will result in a periodic switching between these two comb filter techniques. This leads to a visible periodic flicker in the resolution of the television picture, as explained in more detail below, due to the different resolutions resulting from the use of a line or field comb filter.
To more clearly understand why a conventional motion detector will fail, consider FIG. 1. Firstly, assume that each of the motion picture frames are different. A motion detector comparing corresponding picture elements separated by a frame interval will detect motion between, e.g., corresponding picture elements in fields 4 and 6 (F4/F6). Similarly, it will detect motion between corresponding picture elements in fields F5/F7, F6/F8, and F7/F9. It will not detect motion, however, when it compares picture elements from fields F8/F10 since, although these picture elements are a television frame apart, both these fields correspond to the same motion picture frame (MP 4) and are thus spatially identical. Continuing this evaluation of the motion detector, one can easily see that the motion detector will indicate "no motion" every five television fields although all motion picture frames are different. This wrong decision can lead to the selection of the wrong luma/chroma signal separation algorithm at a regularly occurring low frequency rate, i.e., 12 Hz, and thus result in visually annoying artifacts in the displayed picture due to the difference in signal resolution between line and field comb filtering.
U.S. Pat. No. 4,933,759 uses a plurality of field delay circuits to perform various combinations of signal comparisons in order to develop a motion detection signal. For ease of understanding, the sequential fields (n-2) through (n+3) of FIG. 1 of this patent will be referred to as F1-F6, respectively. Thus, the comparators 1 and 2 of FIG. 2 are responsive to signal in fields F1, F3, F4 and F3, F4, F5, and then F2, F3, F4 and F3, F4, F6, respectively, when switches 1 and 2 are switched to their alternate positions.
Due to the fact that in the above-noted comparisons, adjacent fields are being compared, i.e., fields F3/F4: F3/F4/F5; F2/F3/F4; and F3/F4, and the presence of line interlace, a 4-point averaging technique is required to spatially match a point in one field with a corresponding point in an adjacent field (due to the interlace "line offset").
This necessarily complicates the comparator functions and also, due to the possibility of e.g., a black/white transition occurring in the image space between adjacent field lines, which would result in a gray output from the comparator, the threshold required to indicate motion would have to be set a relatively high value, thereby increasing the chance of an indication of no motion when in fact there was motion.